1. Field of the Invention
The present invention relates to a thermosensitive recording material, in which a protective layer containing a fluorescent whitening agent is formed on a thermosensitive coloring layer, and an image recording method using the thermosensitive recording material.
2. Description of the Related Art
Thermosensitive recording materials are used in various fields: POS field such as perishable foods, boxed meals, and prepared food; copying field such as books and documents; communication field such as facsimile; ticketing field such as ticket vending machines and receipts; and aviation industry such as baggage tags. As the thermosensitive recording materials are widely used, improvement of whiteness, light resistance, and water resistance have been increasingly demanded.
As such thermosensitive recording material, conventionally known thermosensitive recording materials are those having a protective layer containing a fluorescent whitening agent provided on a thermosensitive coloring layer for improving whiteness.
For example, proposed is a thermosensitive recording material having a protective layer which contains 0.1 parts by mass to 10 parts by mass of a fluorescent whitening agent (see Japanese Patent Application Laid-Open (JP-A) No. 62-184880). Moreover, a thermosensitive recording material having a protective layer, which contains a cationic fluorescent naphthalimide dye, has been proposed (Japanese Patent (JP-B) No. 3446092).
However, in these proposals, since the small amount of the fluorescent whitening agent is contained in the protective layer, effect on improvement of whiteness in appearance is confirmed, but light resistance is not sufficiently attained.
It has been known that the light resistance is improved by adding the fluorescent whitening agent. For example, there has been proposed a thermosensitive recording material in which a stilbene fluorescent dye is contained in a dry mass of 0.1 g/m2 to 0.5 g/m2 in a protective layer, which is provided on a thermosensitive coloring layer (JP-B No. 3463080).
According to this proposal, the light resistance is improved to a certain degree, but sufficient light resistance has not been achieved. Namely, such problem is pointed out that when the amount of the fluorescent whitening agent (fluorescent dye) is increased to achieve sufficient light resistance, background is turned into yellow color. In the composition of this proposal, it is difficult to increase the amount of the fluorescent whitening agent more than the aforementioned range.
Similar to this proposal, there has been proposed a method of adding a fluorescent whitening agent to a protective layer (see JP-A No. 10-44609) so as to improve the light resistance. However, there is also a problem that sufficient light resistance cannot be achieved.
On the other hand, there is a proposal of a method of attaining the light resistance by adding a fluorescent whitening agent to a protective layer of a thermosensitive recording material that uses near-infrared absorption material as a coloring material (see JP-A No. 3156401). Here, it has been disclosed an Example that the sufficient light resistance is attained in a thermosensitive recording material, in which a protective layer is formed by using a protective layer dispersion liquid, which is prepared by adding 40 parts by mass of a 50% fluorescent dye aqueous solution to 100 parts by mass of a dispersion liquid.
With this amount of the fluorescent whitening agent in the Example, the light resistance is improved in the image part, but the color of the background is changed into yellow as described in JP-B No. 3463080, and satisfiable background whiteness cannot be obtained.
Moreover, in the thermosensitive recording material in which near-infrared absorption material is used as a coloring material, another method of containing the fluorescent whitening agent in the protective layer has been proposed (JP-A No. 06-48038). In this proposal, a main effect is to improve the whiteness in appearance, and the improvement of the light resistance is not achieved.
Therefore, in these proposals, it is difficult to achieve both the light resistance and the background whiteness.
Moreover, when the certain amount of the fluorescent whitening agent is intended to be provided in the resulting recording material by increasing the deposition amount of the protective layer while the amount of the fluorescent whitening agent is kept low in the protective layer dispersing solution, it is necessary to considerably increase the deposition amount of the protective layer. There is a problem of causing decrease in coloring properties in a thermosensitive coloring layer.
In light of these problems, there has been proposed a method of attaining both the light resistance and the background whiteness by using an ultraviolet absorbing agent in combination with the fluorescent whitening agent so as to decrease the amount of the fluorescent whitening agent (see JP-B Nos. 3829426, 3635388 and 2936556, and JP-A Nos. 10-235996 and 08-282114).
However, in the case where the ultraviolet absorbing agent is contained in a thermosensitive coloring layer, storage stability such as heat resistance and coloring properties decrease, and in the case where the ultraviolet absorbing agent is contained in a protective layer, sticking and poor head matching properties such as contaminant adhesion to a head occur.
The mechanism of deterioration by light is not exactly known. However, it is experimentally confirmed that when the thermosensitive coloring layer is exposed to light such as sunlight in the state that oxygen ingresses in the thermosensitive coloring layer, a leuco dye contained in the thermosensitive coloring layer is degraded by reaction with the oxygen and ultraviolet light contained in the sunlight etc., causing change of color in the background of the thermosensitive recording material and color fading of images thereon.
To drastically improve the light resistance, it is important to block not only the ultraviolet light, but also the oxygen. Particularly, oxygen ingresses not only from the upper side of the thermosensitive coloring layer (i.e. the side exposed to light such as sunlight, etc.), but also from the lower side of the thermosensitive coloring layer. Thus, it is necessary to block oxygen which ingresses from the lower side of the thermosensitive coloring layer as well as from the upper side thereof.
In this point, in prior art, a protective layer is provided on the thermosensitive coloring layer so as to block the oxygen ingression from the upper side, but blocking of the oxygen ingression from the lower side has not been studied. Namely, an undercoat layer is conventionally provided for the purpose of improvement of coloring sensitivity and fineness of printed images. Accordingly, the undercoat layer is designed to have a function of enhancing porosity inside the layer, and of improving adhesion properties and thermal insulation properties of the thermosensitive recording material.
Since the above-mentioned function is inhibited by adding a large amount of a resin in the undercoat layer, only the minimum amount of the resin is used to impart binding properties between layers. As a result, the conventionally used undercoat layer has low ability of blocking oxygen, and satisfiable light resistance cannot be obtained.
For example, in JP-B Nos. 3829426 and 3635388, and JP-A No. 10-235996, there has been proposed to provide an under layer (undercoat layer). Each of these undercoat layers enhances coloring sensitivity and has a function of blocking ultraviolet light. However, as described above, the undercoat layer is designed to increase hollow with decreasing the amount of the resin so as to enhance the coloring sensitivity, causing high air permeance. Consequently, the undercoat layer has low ability of blocking oxygen, and satisfiable light resistance cannot be attained.
Therefore, at the moment, there is no satisfiable thermosensitive recording material having excellent light resistance and water resistance, and high whiteness.